3I/ATLAS: Strange Visitor From Another Star, or Something Else?

When astronomers first spotted 3I/ATLAS in July 2025, it instantly became only the third of its kind ever seen. The only other interstellar objects we’ve recorded were ‘Oumuamua in 2017 and Borisov in 2019. Borisov behaved much like a normal comet, just one that came from outside our Solar System. ‘Oumuamua was different. Its odd shape, strange tumbling, and unexplained acceleration left experts divided. So when 3I/ATLAS appeared, the question came quickly: would it be another ordinary comet, or something harder to explain?

At first, 3I/ATLAS seemed like a comet in every way. Its hyperbolic path showed it hadn’t formed here, and it behaved as we’d expect as it drew closer to the Sun. It grew a tail, shed dust and gas, and glowed green, the sort of thing we expect from a comet. But as astronomers studied it more closely, the details didn’t quite match what we know.

The chemistry was the first surprise. Observations with the James Webb Space Telescope and others showed that its atmosphere, or coma, carried far more carbon dioxide than water. That’s not what we normally see in comets. It pointed to a very different birthplace, maybe in a colder or more unusual region than the one that shaped our own.

Then came its activity. Most comets stay quiet until the Sun warms them up. 3I/ATLAS was already active at much greater distances, with jets of gas likely driven by carbon monoxide and carbon dioxide. It didn’t behave the way most comets do at that distance.

Observers also noticed changes in its glow. Early views showed a reddish tint, which later shifted to green, especially during September’s lunar eclipse. That suggested fresh gases breaking free beneath the surface as its crust broke down. Add to that its retrograde orbit, running opposite to the planets, yet tilted only about five degrees from the plane of the Solar System, and the whole thing started to look less like the comets we know.

As for its size, that’s still uncertain. Hubble data suggest the solid core could be just a few hundred meters across, or as much as five and a half kilometres. Most of what we see is the surrounding cloud and tail. For now, we don’t really know how heavy or dense it is.

All of this could still be natural, but together it raised questions. Some scientists see it as another reminder that comets formed in other star systems won’t always behave like the ones we’re used to. Others, like Avi Loeb, argue that the anomalies are reason enough to at least consider other possibilities, including the idea that it could be artificial.

Not everyone is convinced it’s just a comet, and that has led to some very different theories. Some suggest it could be a derelict alien craft that has drifted for eons before crossing our skies. Others argue that its strange chemistry might be signs of propulsion rather than natural outgassing. A few even go further, calling it a “cosmic messenger” sent our way, or a kind of time capsule holding something from another civilisation. People said the same thing about ‘Oumuamua, that it could be a disguised probe slipping past us, watching but never making itself known.

NASA says there’s no proof it’s anything other than a comet. They point to its tail, its glow, and its changing chemistry as evidence that it’s behaving like one, even if not in the usual way. But when an object from another star doesn’t follow the patterns we know, people naturally look for other explanations. Comets have never just been lumps of rock and ice in the night sky. In ancient times they were seen as omens, warnings of war or change, signs that the sky could spill over into everyday life. Today the language has shifted from prophecy to technology, but the impulse hasn’t changed. When something strange appears above us, we still search for meaning, whether in science, myth, or the possibility of another intelligence watching from afar.

What makes 3I/ATLAS compelling is less about proving one theory right or wrong than what it represents. Each interstellar visitor brings with it a piece of another world. Its unusual chemistry may not mean technology, but it does prove that other star systems build comets in ways that don’t always mirror our own. That, in itself, is remarkable.

As it makes its close pass by Mars and heads toward the Sun later this month, 3I/ATLAS will give us a rare look at something that doesn’t belong here. Whether it ends up as just another comet in the record books or forces us to rethink what interstellar objects can be, it’s a reminder that science and mystery often walk side by side. From ancient omens to talk of alien probes, we’ve always tried to explain the lights above us. And maybe that is the real lesson of 3I/ATLAS, that no matter how far it has traveled, the real gap it crosses is between what we can measure and what we can only imagine.

For skywatchers hoping to see it for themselves, 3I/ATLAS is currently visible through mid-sized telescopes, glowing faintly green in the constellation Pisces as it makes its way past Mars. Its closest approach to Earth will come in December 2025, when observers with good conditions and the right equipment may catch one more glimpse of this traveler before it disappears back into interstellar space.